A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disk having magnetic surfaces. When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head that is positioned over a specific location of a disk by an actuator. A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. Write heads make use of the electricity flowing through a coil, which produces a magnetic field. Electrical pulses are sent to the write head, with different patterns of positive and negative currents. The current in the coil of the head induces a magnetic field across the gap between the head and the magnetic disk, which in turn magnetizes a small area on the recording medium.
The spindle system for rotating the one or more disk typically includes a motor assembly (sometimes referred to as a spindle motor assembly), which includes a bearing assembly. One approach to a bearing assembly is use of a fluid dynamic bearing (FDB), which generally may have a longer life than a ball bearing and is more suitable for relatively high vibration environments and/or low noise requirements. As with many if not all dynamic systems, the motor assembly and in turn the associated bearing assembly are subject to vibration forces and, therefore, have unique vibration force responses. In the context of HDDs, poor vibration response characteristics can cause, for example, a recording disk to bend and/or a disk stack to tilt, and the like. These effects in turn can cause track misregistration (TMR), such as non-repeatable runout (NRRO). Furthermore, customers mandate meeting stringent performance requirements, which may include noise requirements as well as vibration requirements, which generally relate to an HDD's operational resistance to or operational tolerance of vibration forces. Thus, improvements to vibration force responses are a common challenge facing engineers and designers of dynamic systems, systems affected by external vibrations, and the like.
Any approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.